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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 <pmohapat@cumulusnetworks.com>
Reviewed-by: Dinesh G Dutt <ddutt@cumulusnetworks.com>
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| .. | ||
| .gitignore | ||
| BGP4-MIB.txt | ||
| bgp_advertise.c | ||
| bgp_advertise.h | ||
| bgp_aspath.c | ||
| bgp_aspath.h | ||
| bgp_attr.c | ||
| bgp_attr.h | ||
| bgp_btoa.c | ||
| bgp_clist.c | ||
| bgp_clist.h | ||
| bgp_community.c | ||
| bgp_community.h | ||
| bgp_damp.c | ||
| bgp_damp.h | ||
| bgp_debug.c | ||
| bgp_debug.h | ||
| bgp_dump.c | ||
| bgp_dump.h | ||
| bgp_ecommunity.c | ||
| bgp_ecommunity.h | ||
| bgp_filter.c | ||
| bgp_filter.h | ||
| bgp_fsm.c | ||
| bgp_fsm.h | ||
| bgp_main.c | ||
| bgp_mpath.c | ||
| bgp_mpath.h | ||
| bgp_mplsvpn.c | ||
| bgp_mplsvpn.h | ||
| bgp_network.c | ||
| bgp_network.h | ||
| bgp_nexthop.c | ||
| bgp_nexthop.h | ||
| bgp_open.c | ||
| bgp_open.h | ||
| bgp_packet.c | ||
| bgp_packet.h | ||
| bgp_regex.c | ||
| bgp_regex.h | ||
| bgp_route.c | ||
| bgp_route.h | ||
| bgp_routemap.c | ||
| bgp_snmp.c | ||
| bgp_snmp.h | ||
| bgp_table.c | ||
| bgp_table.h | ||
| bgp_vty.c | ||
| bgp_vty.h | ||
| bgp_zebra.c | ||
| bgp_zebra.h | ||
| bgpd.c | ||
| bgpd.conf.sample | ||
| bgpd.conf.sample2 | ||
| bgpd.h | ||
| IMPLEMENTATION.txt | ||
| Makefile.am | ||