diff options
| -rw-r--r-- | doc/bird.sgml | 31 | ||||
| -rw-r--r-- | proto/bgp/attrs.c | 12 | ||||
| -rw-r--r-- | proto/bgp/bgp.h | 1 | ||||
| -rw-r--r-- | proto/bgp/config.Y | 3 |
4 files changed, 34 insertions, 13 deletions
diff --git a/doc/bird.sgml b/doc/bird.sgml index 86f4c507..b35f4944 100644 --- a/doc/bird.sgml +++ b/doc/bird.sgml @@ -1121,7 +1121,7 @@ for each neighbor using the following configuration parameters: BIRD does not implement obsoleted RFC 1863, but uses ad-hoc implementation, which behaves like plain EBGP but reduces modifications to advertised route attributes to be transparent (for example does not prepend its AS number to - AS PATH attribute and keep MED attribute). Default: disabled. + AS PATH attribute and keeps MED attribute). Default: disabled. <tag>enable route refresh <m/switch/</tag> When BGP speaker changes its import filter, it has to re-examine all routes @@ -1205,6 +1205,15 @@ for each neighbor using the following configuration parameters: <tag>path metric <m/switch/</tag> Enable comparison of path lengths when deciding which BGP route is the best one. Default: on. + <tag>med metric <m/switch/</tag> Enable comparison of MED + attributes (during best route selection) even between routes + received from different ASes. This may be useful if all MED + attributes contain some consistent metric, perhaps enforced in + import filters of AS boundary routers. If this option is + disabled, MED attributes are compared only if routes are + received from the same AS (which is the standard behavior). + Default: off. + <tag>igp metric <m/switch/</tag> Enable comparison of internal distances to boundary routers during best route selection. Default: on. @@ -1233,21 +1242,21 @@ with `<tt/O/') are optional. <descrip> <tag>bgppath <cf/bgp_path/</tag> Sequence of AS numbers describing the AS path - the packet will travel through when forwarded according to the particular route. In case of - internal BGP it doesn't contain the number of the local AS. + the packet will travel through when forwarded according to the particular route. + In case of internal BGP it doesn't contain the number of the local AS. <tag>int <cf/bgp_local_pref/ [I]</tag> Local preference value used for selection among multiple BGP routes (see the selection rules above). It's used as an additional metric which is propagated through the whole local AS. <tag>int <cf/bgp_med/ [O]</tag> The Multiple Exit Discriminator of the route - is an optional attribute which is used on on external (inter-AS) links to + is an optional attribute which is used on external (inter-AS) links to convey to an adjacent AS the optimal entry point into the local AS. - The received attribute may be also propagated over internal BGP links - (and this is default behavior). The attribute value is zeroed when a route - is exported from a routing table to a BGP instance to ensure that the attribute - received from a neighboring AS is not propagated to other neighboring ASes. - A new value might be set in the export filter of a BGP instance. + The received attribute is also propagated over internal BGP links. + The attribute value is zeroed when a route is exported to an external BGP + instance to ensure that the attribute received from a neighboring AS is + not propagated to other neighboring ASes. A new value might be set in + the export filter of an external BGP instance. See RFC 4451<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc4451.txt"> for further discussion of BGP MED attribute. @@ -1281,11 +1290,11 @@ with `<tt/O/') are optional. its own routing policy, it also has a complete freedom about which community attributes it defines and what will their semantics be. - <tag>quad <cf/bgp_originator_id/ [O]</tag> This attribute is created by the + <tag>quad <cf/bgp_originator_id/ [I, O]</tag> This attribute is created by the route reflector when reflecting the route and contains the router ID of the originator of the route in the local AS. - <tag>clist <cf/bgp_cluster_list/ [O]</tag> This attribute contains a list + <tag>clist <cf/bgp_cluster_list/ [I, O]</tag> This attribute contains a list of cluster IDs of route reflectors. Each route reflector prepends its cluster ID when reflecting the route. </descrip> diff --git a/proto/bgp/attrs.c b/proto/bgp/attrs.c index e1a3671a..95eee9aa 100644 --- a/proto/bgp/attrs.c +++ b/proto/bgp/attrs.c @@ -1118,7 +1118,17 @@ bgp_rte_better(rte *new, rte *old) return 0; /* RFC 4271 9.1.2.2. c) Compare MED's */ - if (bgp_get_neighbor(new) == bgp_get_neighbor(old)) + /* This is noncompliant. Proper RFC 4271 path selection cannot be + * interpreted as finding the best path in some ordering. + * Therefore, it cannot be implemented in BIRD without some ugly + * hacks. This is just an approximation, which in specific + * situations may lead to persistent routing loops, because it is + * nondeterministic - it depends on the order in which routes + * appeared. But it is also the same behavior as used by default in + * Cisco routers, so it is probably not a big issue. + */ + if (new_bgp->cf->med_metric || old_bgp->cf->med_metric || + (bgp_get_neighbor(new) == bgp_get_neighbor(old))) { x = ea_find(new->attrs->eattrs, EA_CODE(EAP_BGP, BA_MULTI_EXIT_DISC)); y = ea_find(old->attrs->eattrs, EA_CODE(EAP_BGP, BA_MULTI_EXIT_DISC)); diff --git a/proto/bgp/bgp.h b/proto/bgp/bgp.h index b06f20a0..097faa6a 100644 --- a/proto/bgp/bgp.h +++ b/proto/bgp/bgp.h @@ -25,6 +25,7 @@ struct bgp_config { int missing_lladdr; /* What we will do when we don' know link-local addr, see MLL_* */ int gw_mode; /* How we compute route gateway from next_hop attr, see GW_* */ int compare_path_lengths; /* Use path lengths when selecting best route */ + int med_metric; /* Compare MULTI_EXIT_DISC even between routes from differen ASes */ int igp_metric; /* Use IGP metrics when selecting best route */ int prefer_older; /* Prefer older routes according to RFC 5004 */ u32 default_local_pref; /* Default value for LOCAL_PREF attribute */ diff --git a/proto/bgp/config.Y b/proto/bgp/config.Y index e932a7f6..93f832aa 100644 --- a/proto/bgp/config.Y +++ b/proto/bgp/config.Y @@ -25,7 +25,7 @@ CF_KEYWORDS(BGP, LOCAL, NEIGHBOR, AS, HOLD, TIME, CONNECT, RETRY, ADVERTISE, IPV4, CAPABILITIES, LIMIT, PASSIVE, PREFER, OLDER, MISSING, LLADDR, DROP, IGNORE, ROUTE, REFRESH, INTERPRET, COMMUNITIES, BGP_ORIGINATOR_ID, BGP_CLUSTER_LIST, IGP, TABLE, - GATEWAY, DIRECT, RECURSIVE) + GATEWAY, DIRECT, RECURSIVE, MED) CF_GRAMMAR @@ -79,6 +79,7 @@ bgp_proto: | bgp_proto GATEWAY DIRECT ';' { BGP_CFG->gw_mode = GW_DIRECT; } | bgp_proto GATEWAY RECURSIVE ';' { BGP_CFG->gw_mode = GW_RECURSIVE; } | bgp_proto PATH METRIC bool ';' { BGP_CFG->compare_path_lengths = $4; } + | bgp_proto MED METRIC bool ';' { BGP_CFG->med_metric = $4; } | bgp_proto IGP METRIC bool ';' { BGP_CFG->igp_metric = $4; } | bgp_proto PREFER OLDER bool ';' { BGP_CFG->prefer_older = $4; } | bgp_proto DEFAULT BGP_MED expr ';' { BGP_CFG->default_med = $4; } |