User`s guide

Configuring OSPF Routing
Administering OSPF
MultiVoice Gateway for the MAX— User’s Guide Preliminary November 23, 1998 9-15
Multipath routing
A MAX running OSPF can alternate between two equal cost gateways. When OSPF detects
more than one equally good gateway, in terms of routing costs, each equal-cost gateway is put
on an equal-cost list. The router will alternate between all the gateways on the list. This is
called equal-cost multipath routing.
For example, if router A has two equal-cost routes to example.com, one via router B and the
other via router C, the routing table could look like this:
Destination Gateway IF Flg Pref Met Use
Age
10.174.88.0/25 10.174.88.12 wan2 OGM 10 10 52 19
10.174.88.0/25 10.174.88.13 wan3 OGM 10 10 52 19
10.174.88.12/32 10.174.88.12 wan2 OG 10 10 0 28
10.174.88.13/32 10.174.88.13 wan3 OG 10 10 0 28
192.168.253.0/24 - ie0 C 0 0 1 49
192.168.253.6/32 - lo0 CP 0 0 53 49
223.1.1.0/24 10.174.88.12 wan2 OG 10 10 0 19
223.5.1.0/24 10.174.88.12 wan2 OG 10 10 0 19
223.12.9.0/24 10.174.88.12 wan2 OG 10 10 0 19
255.255.255.255/32 - ie0 CP 0 0 0 49
Note that the M in the Flags column indicates an equal-cost multipath. A Traceroute from
router A to example.com would look like this:
ascend% traceroute -q 10 example.com
traceroute to example.com (10.174.88.1), 30 hops max, 0 byte pack-
ets
1 C.example.com (10.174.88.13) 20 ms B .example.com
(10.174.88.12) 20 ms C.example.com (10.174.88.13) 20 ms B .exam-
ple.com (10.174.88.12) 20 ms 20 ms C.example.com (10.174.88.13)
60 ms 20 ms B .example.com (10.174.88.12) 20 ms C.example.com
(10.174.88.13) 20 ms B .example.com (10.174.88.12) 20 ms
2 example.com (10.174.88.1) 20 ms 20 ms 20 ms 20 ms 30 ms 20
ms 20 ms 30 ms 20 ms 30 ms
Note: Notice the alternating replies. The replies are statistically dispatched to B and C, with
roughly 50% of the packets sent through each gateway. (For background information about the
routing table and about the Traceroute command, see Chapter 8, “Configuring IP Routing.”)
Third-party routing
A MultiVoice Gateway running OSPF can advertise routes to external destinations on behalf of
another gateway (a third-party). This is commonly known as advertising a forwarding address.
Depending on the exact topology of the network, other routers might be able to use this type of
LSA and route directly to the forwarding address without involving the advertising MultiVoice
Gateway, thereby increasing the total network throughput.
Third-party routing requires that all OSPF routers know how to route to the forwarding
address. This usually means that the forwarding address must be on an Ethernet that has an
OSPF router acting as the forwarding router, or that the designated router is sending LSAs for
that Ethernet to any area that sees the static route's forwarding-address LSAs.