Riverbed SteelHead 3070-SD, Руководство пользователя

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Configuring BGP, OSPF, Static Routing, and Route Retraction on SteelHead SD | 45
Configuring BGP on SteelHead SD
Configuring BGP path selection
BGP path selection uses a defined set of criteria to determine the most efficient route through a network.
The criteria is listed in the same order in which BGP uses them to select the optimal routes to be injected
into the IP routing table.
1. Highest weight - You configure this value when you configure BGP neighbor (Appliances > BGP tab:
Add BGP Neighbor). A route with a high weight value is preferred among multiple routes with the
same destination. For details, see
“Enabling BGP and configuring BGP neighbors on SteelHead SD”
on page 42 .
2. Highest local preference - you configure this value when you create BGP routing polices (Routing >
Route Maps: Add Route Map). For example, when you create a route map with the use case: Policies
at the BGP neighbor level. The default value for the local preference is 100. If a route has no local
preference specified it is treated as if it had a local preference of 100. If the iBGP speaker receives
multiple routes to the same destination, then the route with the highest value is preferred. For details,
see
“Configuring route maps” on page 76 .
3. Locally generated - Prefer a route that is locally sourced. The best path selection algorithm uses this
criteria to prefer paths that originate locally with a network statement, an aggregate statement or the
redistribution of a route between local routing protocols. Aggregating network routes into one
makes internet routing more efficient by saving network space. You don’t need to configure this
setting because the gateway is able to deduce the information it is generating itself locally rather than
learning it from a nonlocal source. This data is never passed outside of the local route selection
process and is purely internal to the local router.
4. Shortest AS path - You configure this value when you create BGP routing polices (Routing > Route
Maps: Add Route Map). For example, when you create a route map with the use case: Default route
origination in BGP for a neighbor. For details, see
“Creating routing AS path lists” on page 74 and
“Configuring route maps” on page 76 .
5. Origin type - You configure this value when you create BGP routing polices (Routing > Route Maps:
Add Route Map). For example, when you create a route map with the use case: Policies at the BGP
neighbor level. The lowest origin type is the preferred path: IGP is lower than EGP, and EGP is lower
than Incomplete. Only the routes with the lowest origin value are considered when multiple routes
share the shortest AS path, then the algorithm continues by considering the multi exit discriminator
(MED) settings. For details, see
“Configuring route maps” on page 76 .
6. Lowest MED - You configure this value when you configure BGP (Appliances > BGP tab). MED is a
BGP path attribute that can influence the route selection process. MED breaks the tie between the
two routes when the weight, local preference, AS-Path, and origin type are same.
“Enabling multi exit
discriminator (MED) settings” on page 51 .
7. eBGP over iBGP - External BGP routes are preferred over internal BGP routes. iBGP or internal BGP
runs within the same AS, whereas eBGP or external BGP operates between autonomous systems.
8. IBP metric - You configure this value when you create BGP routing polices (Routing > Route Maps:
Add Route Map). For example, when you create a route map with the use case: Policies at the BGP
neighbor level. If there is no external route selection path, the path with the lowest IGP value to the
next hop is preferred. The IGP route is interior to the AS of origination. For details, see
“Configuring
route maps” on page 76 .
9. Route age - Routes with a longer age are preferred over “newer” routes or routes with a shorter age