[updated Pradosh's email address on the cc]
I think this was a case of Pradosh being cautious in his wording when he
wrote this patch. I cannot think of any scenario where this would cause a
routing loop and we have not had any customers have issues with routing
loops. The same used to be said about deterministic-med ("deploy it
everywhere at the same time") but that is another one where I have never
seen a scenario where deploying it in some places and not others actually
causes a loop. For me it is kind of like saying "deploy the same
route-maps everywhere else you are running a different decision algorithm
everywhere and you could get loops" which we know is not the case.
Routing loops in BGP are typically caused when you have one node that is
forwarding down a path but is not advertising that path to his peers (ie
multipath scenario and the whole multipath-relax knob). The proposed
bestpath change does not cause that to happen....I think it is a safe
change.
Daniel
On Tue, Nov 10, 2015 at 4:05 AM, Martin Winter <
[email protected]> wrote:
> Donald / Pradosh,
>
> the segment about the partial deployment causing potential routing loops
> worries me.
>
> What happens on deployments with mixed equipment? Unfortunately not
> everything is running (the latest version of) Quagga yet.
> What is expected if some of these routers are from Cisco? I assume
> this would cause potential loops.
>
> Can we make this configurable?
>
> Regards,
> Martin Winter
> [email protected]
>
>
>
> On 9 Nov 2015, at 17:14, Donald Sharp wrote:
>
> From: Pradosh Mohapatra <[email protected]>
>>
>> A fat tree topology running IBGP gets into two issues with anycast address
>> routing. Consider the following topology:
>>
>> R9 R10
>> x x
>> R3 R4 R7 R8
>> x x
>> R1 R2 R5 R6
>> | | | |
>> 10/8 10/8 10/8 S
>>
>> Let's remind ourselves of BGP decision process steps:
>>
>> 1. Highest Local Preference
>> 2. Shortest AS Path Length
>> 3. Lowest Origin Type
>> 4. Lowest MED (Multi-Exit Discriminator)
>> 5. Prefer External to Internal
>> 6. Closest Egress (Lowest IGP Distance)
>> 7. Tie Breaking (Lowest-Router-ID)
>> 8. Tie Breaking (Lowest-cluster-list length)
>> 9. Tie Breaking (Lowest-neighbor-address)
>>
>> Without any policies, steps 1-6 will almost always evaluate identically
>> for
>> all paths received on any router in the above topology. Let's assume that
>> the router-ids follow the following inequality: R1 < R2 < R5 < R6. Owing
>> to
>> the 7th step above, all routers will now choose R1's path as the best.
>> This
>> is undesirable. As an example, traffic from S to 10/8 will follow the path
>> S -> R6 -> R7 -> R9 -> R4 -> R2 -> 10/8 instead of S -> R6 -> R7 -> R5 ->
>> 10/8.
>> Furthermore, once R7 (& R8) chooses R1's path as the best, it would
>> withdraw
>> its path learned through (R5, R6) from (R9, R10). This leads to
>> inefficient
>> load balancing - e.g. R9 can't do ECMP across all available egresses -
>> (R1, R2, R5).
>>
>> The patch addresses these issues by noting that that cluster list is
>> always
>> carried along with the routes and its length is a good indicator of IBGP
>> hops. It thus makes sense to compare that as an extension to metric after
>> step 6. That automatically ensures correct multipath computation.
>>
>> Unfortunately a partial deployment of this in a generic topology (note:
>> fat-tree/clos topologies work fine) may lead to potential loops. It needs
>> to be looked into.
>>
>> Signed-off-by: Pradosh Mohapatra <[email protected]>
>> Reviewed-by: Dinesh G Dutt <[email protected]>
>> ---
>> bgpd/bgp_attr.h | 4 ++
>> bgpd/bgp_mpath.c | 4 +-
>> bgpd/bgp_mpath.h | 3 +-
>> bgpd/bgp_route.c | 35 ++++++++----
>> bgpd/bgp_vty.c | 140
>> ++++++++++++++++++++++++------------------------
>> bgpd/bgpd.h | 2 +
>> tests/bgp_mpath_test.c | 4 +-
>> 7 files changed, 110 insertions(+), 82 deletions(-)
>>
>> diff --git a/bgpd/bgp_attr.h b/bgpd/bgp_attr.h
>> index b59fa8e..ac346e5 100644
>> --- a/bgpd/bgp_attr.h
>> +++ b/bgpd/bgp_attr.h
>> @@ -132,6 +132,10 @@ struct transit
>>
>> #define ATTR_FLAG_BIT(X) (1 << ((X) - 1))
>>
>> +#define BGP_CLUSTER_LIST_LENGTH(attr) \
>> + (((attr)->flag & ATTR_FLAG_BIT(BGP_ATTR_CLUSTER_LIST)) ? \
>> + (attr)->extra->cluster->length : 0)
>> +
>> typedef enum {
>> BGP_ATTR_PARSE_PROCEED = 0,
>> BGP_ATTR_PARSE_ERROR = -1,
>> diff --git a/bgpd/bgp_mpath.c b/bgpd/bgp_mpath.c
>> index 7999d16..5590b05 100644
>> --- a/bgpd/bgp_mpath.c
>> +++ b/bgpd/bgp_mpath.c
>> @@ -46,7 +46,7 @@
>> */
>> int
>> bgp_maximum_paths_set (struct bgp *bgp, afi_t afi, safi_t safi,
>> - int peertype, u_int16_t maxpaths)
>> + int peertype, u_int16_t maxpaths, u_int16_t
>> options)
>> {
>> if (!bgp || (afi >= AFI_MAX) || (safi >= SAFI_MAX))
>> return -1;
>> @@ -55,6 +55,7 @@ bgp_maximum_paths_set (struct bgp *bgp, afi_t afi,
>> safi_t safi,
>> {
>> case BGP_PEER_IBGP:
>> bgp->maxpaths[afi][safi].maxpaths_ibgp = maxpaths;
>> + bgp->maxpaths[afi][safi].ibgp_flags |= options;
>> break;
>> case BGP_PEER_EBGP:
>> bgp->maxpaths[afi][safi].maxpaths_ebgp = maxpaths;
>> @@ -82,6 +83,7 @@ bgp_maximum_paths_unset (struct bgp *bgp, afi_t afi,
>> safi_t safi,
>> {
>> case BGP_PEER_IBGP:
>> bgp->maxpaths[afi][safi].maxpaths_ibgp = BGP_DEFAULT_MAXPATHS;
>> + bgp->maxpaths[afi][safi].ibgp_flags = 0;
>> break;
>> case BGP_PEER_EBGP:
>> bgp->maxpaths[afi][safi].maxpaths_ebgp = BGP_DEFAULT_MAXPATHS;
>> diff --git a/bgpd/bgp_mpath.h b/bgpd/bgp_mpath.h
>> index 37b9ac8..0645e6c 100644
>> --- a/bgpd/bgp_mpath.h
>> +++ b/bgpd/bgp_mpath.h
>> @@ -49,7 +49,8 @@ struct bgp_info_mpath
>> };
>>
>> /* Functions to support maximum-paths configuration */
>> -extern int bgp_maximum_paths_set (struct bgp *, afi_t, safi_t, int,
>> u_int16_t);
>> +extern int bgp_maximum_paths_set (struct bgp *, afi_t, safi_t, int,
>> u_int16_t,
>> + u_int16_t);
>> extern int bgp_maximum_paths_unset (struct bgp *, afi_t, safi_t, int);
>>
>> /* Functions used by bgp_best_selection to record current
>> diff --git a/bgpd/bgp_route.c b/bgpd/bgp_route.c
>> index 316fa5a..24c17d6 100644
>> --- a/bgpd/bgp_route.c
>> +++ b/bgpd/bgp_route.c
>> @@ -324,7 +324,7 @@ bgp_med_value (struct attr *attr, struct bgp *bgp)
>> /* Compare two bgp route entity. br is preferable then return 1. */
>> static int
>> bgp_info_cmp (struct bgp *bgp, struct bgp_info *new, struct bgp_info
>> *exist,
>> - int *paths_eq)
>> + int *paths_eq, struct bgp_maxpaths_cfg *mpath_cfg)
>> {
>> struct attr *newattr, *existattr;
>> struct attr_extra *newattre, *existattre;
>> @@ -477,6 +477,26 @@ bgp_info_cmp (struct bgp *bgp, struct bgp_info *new,
>> struct bgp_info *exist,
>> if (newm > existm)
>> ret = 0;
>>
>> + /* 8.1. Same IGP metric. Compare the cluster list length as
>> + representative of IGP hops metric. Rewrite the metric value
>> + pair (newm, existm) with the cluster list length. Prefer the
>> + path with smaller cluster list length. */
>> + if (newm == existm)
>> + {
>> + if (peer_sort (new->peer) == BGP_PEER_IBGP
>> + && peer_sort (exist->peer) == BGP_PEER_IBGP
>> + && CHECK_FLAG (mpath_cfg->ibgp_flags,
>> + BGP_FLAG_IBGP_MULTIPATH_SAME_CLUSTERLEN))
>> + {
>> + newm = BGP_CLUSTER_LIST_LENGTH(new->attr);
>> + existm = BGP_CLUSTER_LIST_LENGTH(exist->attr);
>> + if (newm < existm)
>> + ret = 1;
>> + if (newm > existm)
>> + ret = 0;
>> + }
>> + }
>> +
>> /* 9. Maximum path check. */
>> if (newm == existm)
>> {
>> @@ -544,12 +564,8 @@ bgp_info_cmp (struct bgp *bgp, struct bgp_info *new,
>> struct bgp_info *exist,
>> return 0;
>>
>> /* 12. Cluster length comparision. */
>> - new_cluster = exist_cluster = 0;
>> -
>> - if (newattr->flag & ATTR_FLAG_BIT(BGP_ATTR_CLUSTER_LIST))
>> - new_cluster = newattre->cluster->length;
>> - if (existattr->flag & ATTR_FLAG_BIT(BGP_ATTR_CLUSTER_LIST))
>> - exist_cluster = existattre->cluster->length;
>> + new_cluster = BGP_CLUSTER_LIST_LENGTH(new->attr);
>> + exist_cluster = BGP_CLUSTER_LIST_LENGTH(exist->attr);
>>
>> if (new_cluster < exist_cluster)
>> return 1;
>> @@ -1377,7 +1393,8 @@ bgp_best_selection (struct bgp *bgp, struct
>> bgp_node *rn,
>> {
>> if (CHECK_FLAG (ri2->flags, BGP_INFO_SELECTED))
>> old_select = ri2;
>> - if (bgp_info_cmp (bgp, ri2, new_select, &paths_eq))
>> + if (bgp_info_cmp (bgp, ri2, new_select, &paths_eq,
>> + mpath_cfg))
>> {
>> bgp_info_unset_flag (rn, new_select,
>> BGP_INFO_DMED_SELECTED);
>> new_select = ri2;
>> @@ -1436,7 +1453,7 @@ bgp_best_selection (struct bgp *bgp, struct
>> bgp_node *rn,
>> bgp_info_unset_flag (rn, ri, BGP_INFO_DMED_CHECK);
>> bgp_info_unset_flag (rn, ri, BGP_INFO_DMED_SELECTED);
>>
>> - if (bgp_info_cmp (bgp, ri, new_select, &paths_eq))
>> + if (bgp_info_cmp (bgp, ri, new_select, &paths_eq, mpath_cfg))
>> {
>> if (do_mpath && bgp_flag_check (bgp,
>> BGP_FLAG_DETERMINISTIC_MED))
>> bgp_mp_dmed_deselect (new_select);
>> diff --git a/bgpd/bgp_vty.c b/bgpd/bgp_vty.c
>> index 3f2c49a..0bd2ebe 100644
>> --- a/bgpd/bgp_vty.c
>> +++ b/bgpd/bgp_vty.c
>> @@ -651,28 +651,41 @@ DEFUN (no_bgp_confederation_peers,
>> return CMD_SUCCESS;
>> }
>>
>> -/* Maximum-paths configuration */
>> -DEFUN (bgp_maxpaths,
>> - bgp_maxpaths_cmd,
>> - "maximum-paths <1-255>",
>> - "Forward packets over multiple paths\n"
>> - "Number of paths\n")
>> +/**
>> + * Central routine for maximum-paths configuration.
>> + * @peer_type: BGP_PEER_EBGP or BGP_PEER_IBGP
>> + * @set: 1 for setting values, 0 for removing the max-paths config.
>> + */
>> +static int
>> +bgp_maxpaths_config_vty (struct vty *vty, int peer_type, const char
>> *mpaths,
>> + u_int16_t options, int set)
>> {
>> struct bgp *bgp;
>> - u_int16_t maxpaths;
>> + u_int16_t maxpaths = 0;
>> int ret;
>> + afi_t afi;
>> + safi_t safi;
>>
>> bgp = vty->index;
>> + afi = bgp_node_afi (vty);
>> + safi = bgp_node_safi (vty);
>>
>> - VTY_GET_INTEGER_RANGE ("maximum-paths", maxpaths, argv[0], 1, 255);
>> + if (set)
>> + {
>> + VTY_GET_INTEGER_RANGE ("maximum-paths", maxpaths, mpaths, 1, 255);
>> + ret = bgp_maximum_paths_set (bgp, afi, safi, peer_type, maxpaths,
>> + options);
>> + }
>> + else
>> + ret = bgp_maximum_paths_unset (bgp, afi, safi, peer_type);
>>
>> - ret = bgp_maximum_paths_set (bgp, bgp_node_afi (vty),
>> bgp_node_safi(vty),
>> - BGP_PEER_EBGP, maxpaths);
>> if (ret < 0)
>> {
>> vty_out (vty,
>> - "%% Failed to set maximum-paths %u for afi %u, safi %u%s",
>> - maxpaths, bgp_node_afi (vty), bgp_node_safi(vty),
>> VTY_NEWLINE);
>> + "%% Failed to %sset maximum-paths %s %u for afi %u, safi
>> %u%s",
>> + (set == 1) ? "" : "un",
>> + (peer_type == BGP_PEER_EBGP) ? "ebgp" : "ibgp",
>> + maxpaths, afi, safi, VTY_NEWLINE);
>> return CMD_WARNING;
>> }
>>
>> @@ -684,6 +697,17 @@ DEFUN (bgp_maxpaths,
>> return CMD_SUCCESS;
>> }
>>
>> +
>> +/* Maximum-paths configuration */
>> +DEFUN (bgp_maxpaths,
>> + bgp_maxpaths_cmd,
>> + "maximum-paths <1-255>",
>> + "Forward packets over multiple paths\n"
>> + "Number of paths\n")
>> +{
>> + return bgp_maxpaths_config_vty(vty, BGP_PEER_EBGP, argv[0], 0, 1);
>> +}
>> +
>> DEFUN (bgp_maxpaths_ibgp,
>> bgp_maxpaths_ibgp_cmd,
>> "maximum-paths ibgp <1-255>",
>> @@ -691,30 +715,19 @@ DEFUN (bgp_maxpaths_ibgp,
>> "iBGP-multipath\n"
>> "Number of paths\n")
>> {
>> - struct bgp *bgp;
>> - u_int16_t maxpaths;
>> - int ret;
>> -
>> - bgp = vty->index;
>> -
>> - VTY_GET_INTEGER_RANGE ("maximum-paths", maxpaths, argv[0], 1, 255);
>> -
>> - ret = bgp_maximum_paths_set (bgp, bgp_node_afi (vty),
>> bgp_node_safi(vty),
>> - BGP_PEER_IBGP, maxpaths);
>> - if (ret < 0)
>> - {
>> - vty_out (vty,
>> - "%% Failed to set maximum-paths ibgp %u for afi %u, safi
>> %u%s",
>> - maxpaths, bgp_node_afi (vty), bgp_node_safi(vty),
>> VTY_NEWLINE);
>> - return CMD_WARNING;
>> - }
>> -
>> - if ((MULTIPATH_NUM != 0) && (maxpaths > MULTIPATH_NUM))
>> - vty_out (vty,
>> - "%% Warning: maximum-paths set to %d is greater than %d that
>> zebra is compiled to support%s",
>> - maxpaths, MULTIPATH_NUM, VTY_NEWLINE);
>> + return bgp_maxpaths_config_vty(vty, BGP_PEER_IBGP, argv[0], 0, 1);
>> +}
>>
>> - return CMD_SUCCESS;
>> +DEFUN (bgp_maxpaths_ibgp_cluster,
>> + bgp_maxpaths_ibgp_cluster_cmd,
>> + "maximum-paths ibgp <1-255> equal-cluster-length",
>> + "Forward packets over multiple paths\n"
>> + "iBGP-multipath\n"
>> + "Number of paths\n"
>> + "Match the cluster length\n")
>> +{
>> + return bgp_maxpaths_config_vty(vty, BGP_PEER_IBGP, argv[0],
>> + BGP_FLAG_IBGP_MULTIPATH_SAME_CLUSTERLEN,
>> 1);
>> }
>>
>> DEFUN (no_bgp_maxpaths,
>> @@ -724,22 +737,7 @@ DEFUN (no_bgp_maxpaths,
>> "Forward packets over multiple paths\n"
>> "Number of paths\n")
>> {
>> - struct bgp *bgp;
>> - int ret;
>> -
>> - bgp = vty->index;
>> -
>> - ret = bgp_maximum_paths_unset (bgp, bgp_node_afi (vty),
>> bgp_node_safi(vty),
>> - BGP_PEER_EBGP);
>> - if (ret < 0)
>> - {
>> - vty_out (vty,
>> - "%% Failed to unset maximum-paths for afi %u, safi %u%s",
>> - bgp_node_afi (vty), bgp_node_safi(vty), VTY_NEWLINE);
>> - return CMD_WARNING;
>> - }
>> -
>> - return CMD_SUCCESS;
>> + return bgp_maxpaths_config_vty(vty, BGP_PEER_EBGP, NULL, 0, 0);
>> }
>>
>> ALIAS (no_bgp_maxpaths,
>> @@ -757,22 +755,7 @@ DEFUN (no_bgp_maxpaths_ibgp,
>> "iBGP-multipath\n"
>> "Number of paths\n")
>> {
>> - struct bgp *bgp;
>> - int ret;
>> -
>> - bgp = vty->index;
>> -
>> - ret = bgp_maximum_paths_unset (bgp, bgp_node_afi (vty),
>> bgp_node_safi(vty),
>> - BGP_PEER_IBGP);
>> - if (ret < 0)
>> - {
>> - vty_out (vty,
>> - "%% Failed to unset maximum-paths ibgp for afi %u, safi
>> %u%s",
>> - bgp_node_afi (vty), bgp_node_safi(vty), VTY_NEWLINE);
>> - return CMD_WARNING;
>> - }
>> -
>> - return CMD_SUCCESS;
>> + return bgp_maxpaths_config_vty(vty, BGP_PEER_IBGP, NULL, 0, 0);
>> }
>>
>> ALIAS (no_bgp_maxpaths_ibgp,
>> @@ -783,6 +766,15 @@ ALIAS (no_bgp_maxpaths_ibgp,
>> "iBGP-multipath\n"
>> "Number of paths\n")
>>
>> +ALIAS (no_bgp_maxpaths_ibgp,
>> + no_bgp_maxpaths_ibgp_cluster_cmd,
>> + "no maximum-paths ibgp <1-255> equal-cluster-length",
>> + NO_STR
>> + "Forward packets over multiple paths\n"
>> + "iBGP-multipath\n"
>> + "Number of paths\n"
>> + "Match the cluster length\n")
>> +
>> int
>> bgp_config_write_maxpaths (struct vty *vty, struct bgp *bgp, afi_t afi,
>> safi_t safi, int *write)
>> @@ -797,8 +789,12 @@ bgp_config_write_maxpaths (struct vty *vty, struct
>> bgp *bgp, afi_t afi,
>> if (bgp->maxpaths[afi][safi].maxpaths_ibgp != BGP_DEFAULT_MAXPATHS)
>> {
>> bgp_config_write_family_header (vty, afi, safi, write);
>> - vty_out (vty, " maximum-paths ibgp %d%s",
>> - bgp->maxpaths[afi][safi].maxpaths_ibgp, VTY_NEWLINE);
>> + vty_out (vty, " maximum-paths ibgp %d",
>> + bgp->maxpaths[afi][safi].maxpaths_ibgp);
>> + if (CHECK_FLAG (bgp->maxpaths[afi][safi].ibgp_flags,
>> + BGP_FLAG_IBGP_MULTIPATH_SAME_CLUSTERLEN))
>> + vty_out (vty, " equal-cluster-length");
>> + vty_out (vty, "%s", VTY_NEWLINE);
>> }
>>
>> return 0;
>> @@ -9183,14 +9179,20 @@ bgp_vty_init (void)
>> install_element (BGP_IPV6_NODE, &no_bgp_maxpaths_cmd);
>> install_element (BGP_IPV6_NODE, &no_bgp_maxpaths_arg_cmd);
>> install_element (BGP_NODE, &bgp_maxpaths_ibgp_cmd);
>> + install_element(BGP_NODE, &bgp_maxpaths_ibgp_cluster_cmd);
>> install_element (BGP_NODE, &no_bgp_maxpaths_ibgp_cmd);
>> install_element (BGP_NODE, &no_bgp_maxpaths_ibgp_arg_cmd);
>> + install_element (BGP_NODE, &no_bgp_maxpaths_ibgp_cluster_cmd);
>> install_element (BGP_IPV4_NODE, &bgp_maxpaths_ibgp_cmd);
>> + install_element(BGP_IPV4_NODE, &bgp_maxpaths_ibgp_cluster_cmd);
>> install_element (BGP_IPV4_NODE, &no_bgp_maxpaths_ibgp_cmd);
>> + install_element (BGP_IPV4_NODE, &no_bgp_maxpaths_ibgp_cluster_cmd);
>> install_element (BGP_IPV4_NODE, &no_bgp_maxpaths_ibgp_arg_cmd);
>> install_element (BGP_IPV6_NODE, &bgp_maxpaths_ibgp_cmd);
>> + install_element(BGP_IPV6_NODE, &bgp_maxpaths_ibgp_cluster_cmd);
>> install_element (BGP_IPV6_NODE, &no_bgp_maxpaths_ibgp_cmd);
>> install_element (BGP_IPV6_NODE, &no_bgp_maxpaths_ibgp_arg_cmd);
>> + install_element (BGP_IPV6_NODE, &no_bgp_maxpaths_ibgp_cluster_cmd);
>>
>> /* "timers bgp" commands. */
>> install_element (BGP_NODE, &bgp_timers_cmd);
>> diff --git a/bgpd/bgpd.h b/bgpd/bgpd.h
>> index 7ae0acb..f5e6936 100644
>> --- a/bgpd/bgpd.h
>> +++ b/bgpd/bgpd.h
>> @@ -171,6 +171,8 @@ struct bgp
>> struct bgp_maxpaths_cfg {
>> u_int16_t maxpaths_ebgp;
>> u_int16_t maxpaths_ibgp;
>> + u_int16_t ibgp_flags;
>> +#define BGP_FLAG_IBGP_MULTIPATH_SAME_CLUSTERLEN (1 << 0)
>> } maxpaths[AFI_MAX][SAFI_MAX];
>> };
>>
>> diff --git a/tests/bgp_mpath_test.c b/tests/bgp_mpath_test.c
>> index 3594753..812c1f6 100644
>> --- a/tests/bgp_mpath_test.c
>> +++ b/tests/bgp_mpath_test.c
>> @@ -157,9 +157,9 @@ run_bgp_cfg_maximum_paths (testcase_t *t)
>> for (safi = SAFI_UNICAST; safi < SAFI_MAX; safi++)
>> {
>> /* test bgp_maximum_paths_set */
>> - api_result = bgp_maximum_paths_set (bgp, afi, safi,
>> BGP_PEER_EBGP, 10);
>> + api_result = bgp_maximum_paths_set (bgp, afi, safi,
>> BGP_PEER_EBGP, 10, 0);
>> EXPECT_TRUE (api_result == 0, test_result);
>> - api_result = bgp_maximum_paths_set (bgp, afi, safi,
>> BGP_PEER_IBGP, 10);
>> + api_result = bgp_maximum_paths_set (bgp, afi, safi,
>> BGP_PEER_IBGP, 10, 0);
>> EXPECT_TRUE (api_result == 0, test_result);
>> EXPECT_TRUE (bgp->maxpaths[afi][safi].maxpaths_ebgp == 10,
>> test_result);
>> EXPECT_TRUE (bgp->maxpaths[afi][safi].maxpaths_ibgp == 10,
>> test_result);
>> --
>> 1.7.10.4
>>
>>
>> _______________________________________________
>> Quagga-dev mailing list
>> [email protected]
>> https://lists.quagga.net/mailman/listinfo/quagga-dev
>>
>
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